This invention relates to some improvements to rotary sheeters to suit them to the cut of wood veneers. Are known rotary sheeters, usually employed for cutting paper and cardboard sheets, that are provided with a straight blade having two or more parallel cutting edges, rotating around its longitudinal axis, and in which each cutting edge alternatively operates with the other through the rotation of the blade along an arc comprised between two successive cutting edges, that in case when a double edged blade is a 180.degree. arc. Such blade cooperates with a holding up roller on which the sheets to be cut are advanced from a feeding conveyor belt and after the cut are moved away by means of a running conveyor belt, so that the minimum length of the cut sheets correspondes to the longitudinal development of the arc comprised between two successive cutting edges, because the peripheral speed of the blade is the same as that of the feeding conveyor belt and of the holding up roller. The above mentioned rotary sheeters are also equipped with means for automatic operation. To this purpose a device (scanner) is installed over the feeding conveyor belt, with the particular function to detect and signal possible defects present in the materials to an electronic apparatus storing the received data, returning them at the right moment for the cut and taking into account both the distance between the scanner and the blade and the length of the arc described by the rotating blade between two successive cuts.
In order to perform correct and timely cuts, the electronic apparatus is connected to an incremental pulse generator which exactly measures the length of the sheets running by and, at the right time, operates the engagement of a friction clutch, that rotates the blade of an angle comprised between two successive cutting edges, that is an angle of 180.degree.. Such friction clutch is driven by a motor that synchronically controls the rotation both of the holding up and of the conveyor belt driving rollers.
In practice, however, the above mentioned rotary sheeters proved to be effective only for cutting sheet materials without imperfections, such as paper sheets and cardboard sheets; on the contrary they present considerable disadvantages in cutting wood veneers. This fact depends on the natural structure of this material. Indeed, due to the presence of knots, checks and other imperfections in the tree stock, the successive cuts of wood veneers must be irregularly spaced to exclude the imperfect sections.
When the well-known rotary sheeters are used, it is evident that the veneers can not be cut in sections having a length shorter than that of the arc described by the edged blade, so that the veneers, cut according to such length, must be cut again to exclude the sections containing imperfections. In this way the perfect sections are saved and an excessive waste of materials is avoided (the minimum length permitted by the double edged blades is comprised in the range of about 22-25 cm.). Of course, the further veneer section cuts constitute expensive and time-consuming operations that affect the productivity of the rotary sheeters which otherwise could operate at relatively high speed.